AC line voltage supplied power systems with reserve power capability normally have an inverter energized by DC voltage from rectified AC voltage. Reserve energy, to operate the inverter, is supplied by a battery. Either DC voltage source energizes the primary inverters and a reserve inverter to generate a power signal with the desired waveform. Control circuitry is required to both control the AC voltage waveform generated by the inverter so that it is in synchronism, is substantially identical to the desired voltage and to substitute the reserve inverter AC voltage, for the desired voltage should there be a failure of either the inverter circuitry or of synchronizing circuitry.